Selective transmission apparatus



May 21, 1968 H. F. CHAPELL 3 5 SELECTIVE TRANSMISSION AFPARATUS Filed Dec. 4. 1964 INSERTION-db 1 /NVENTOR NJ HARRY E CHAPELL 400 5@ 600 700 800 900 1000 FREQUENCY -Mc ATTOI-?NEYS United States Patent O 3,384,843 SELECTIVE TRANSMISSION APPARATUS Harry F. Chapell, Maynard, Mass., assignor to Sage Laboratories, Inc., East Natiek, Mass., a corporation of Massachusetts Filed Dec. 4, 1964, Ser. No. 415,910 7 Claims. (Cl. 333-73) ABSTRACT OF THE- DISCLOSURE An interdigital filter includes an inner conductor and an outer conductor defining an annular cavity between a pair of end plates. A conducting plate extends between the end plates in a radial plane between the inner and outer conductors and is immediately adjacent to and separates input and output coaxial terminal pairs, one pair being in one end plate and the other pair being in the other end plate so that energy exchanged between the terminal pairs travels circumferentially about the inner conductor along a path whose projection on any of the end plates is subtended by an angle of slightly less than 360 with respect to a vertex coincident with the common axis of the inner and outer conductors. The annular Volume between inner and outer conductors includes a number of conducting rods generally parallel to the inner conductor for coacting with the rest of the structure to provide a desired frequency sensitive transmission characteristic.

The present invention relates in general to selective transmission and more particularly concerns novel means for selectvely transmitting high frequency energy over a relatively wide frequency range with little attenuation while sharply increasing the degree of attenuation immediately outside the desired energy transmission band. The desired selective transmission is accomplished with compact apparatus relatively easy and inexpensive to fabricate with input and output terminals conveniently arranged for coupling to external apparatus.

It is an important object of this invention to provide a structure for selectively transmitting energy that is relatively compact and easy to construct while exhibiting excellent electrical performance characteristics.

It is another object of the invention to achieve the preceding object with apparatus that transmits energy over a relatively wide band of frequencies with very little and nearly constant attenuation while imparting a high degree of attenuation to frequencies just outside the desired transmission band.

It is a further object of the invention to achieve the preceding objects with apparatus that negligibly interferes with the VSWR of a system to which it is connected for energy of frequencies within its transmission band.

It is still another object of the invention to achieve the preceding objects with a compact structure having coaxial input and output terminal pairs.

According to the invention, the selective transmission apparatus comprises conducting means defining an annular cavity bounded by coaxial inner and outer conductors separating first and second end plates. A conducting plate intercouples the inner and outer conductors and resides in a radial plane extending between the first and second end plates. First and second coaxial terminal pairs in the first and second end plates, respectively, are intercoupled by means including conducting rods within the annular cavity generally parallel to the cavity axis. Preferably, the separation between the end plates corresponds substantially to a quarter wavelength or less at a frequency within the band of Wavelengths transmitted between the "ice first and second coaxial terminal pairs. By adding capacitance between the ends of the conducting rods and the opposed end plates, the distance between opposed end plates can be reduced substantially below a quarter wavelength. Numerous other features, objects and advantages of the invention will become apparent from the following specification when read in connection with the accompanying drawing in which:

FIG. 1 is a longitudinal sectional view through section 11 of FIG. 2 of an exemplary embodiment of the invention;

FIG. 2 is an end view of the exemplary embodiment;

FIG. 3 is a partially exploded perspective view of the exemplary embodiment with portions of the outer coaxial conductor removed to better illustrate the internal structure;

FIG. 4 is a side view of the exemplary embodiment with a portion of the outer conducting wall cut away to illustrate the position of the conducting rod connected to one of the coaxial terminal pair inner terminals; and

FIG. 5 is a graphical representation of insertion loss as a function of frequency for an exemplary embodiment of the invention in which the upper frequency of the passband is nearly 1.8 times the lower frequency.

With reference now to the drawing and more particularly FIG. l thereof, there is shown a lengthwise sectonal view of an exernplary embodiment of the invention. Energy is selectively transmitted between terminals 11 and 12 with very little attenuation in the passband and considerable attenuation just outside the passband.

An inner conductor 13 coaxial with an outer conductor 14 defines an annular cavity between end plates 15 and 16. A conducting plate 17 extends between end plates 15 and 16 in a radial plane between the inner and outer coaxial conductors 13 and 14 respectively. The plane of conducting plate 17 is immediately adjacent to and separates coaxial terminal pairs 11 and 12 so that energy exchanged between terminals 11 and 12 travels circumferentially about inner conductor 13 along a path whose projection on any of the end plates is subtended by an angle of slightly less than 360 with respect to a vertex coincident with the common axis of inner conductor 13 and outer conductor 14.

The annular Volume between inner conductor 13 and outer conductor 14 includes a number of conducting rods generally parallel to inner conductor 13. As best seen in FIG. 4 a first of these rods 21 is formed with a recess 22 at one end for accommodating the inner terminal 23 of coaxial input terminal pair 11. The other end of the rod 21 resides in a cup-shaped insulator 24 supported in a mating recess in end plate 16. An annular insulating sleeve 25 supports inner terminal 23 and insulatedly separates it from outer terminal 26 of coaxial terminal pair 11. A conducting rod 31 is similarly supported between an nsulating support 32 supported in end plate 15 and the inner terminal 33 of coaxial terminal pair 12.

SiX other rods are situated in the annular region generally parallel to center conductor 13. conducting rods 34, 35 and 36 are connected to and supported in end plate 16 while rods 41, 42 and 43 are connected to and supported in end plate 15. These six conducting rods dovetail, as best seen in FIG. 3, and are symmetrically disposed about the plane defined by conducting plate 17. That is, a projection of each rod connected to end plate 16 corresponds to the mirror image of a projection of a rod connected to end plate 15 about the plate defined by conducting plate 17. This symmetrcal disposition results in essentially equal input and output impedances presented at terminals 11 and 12.

In the specific embodiment shown, each of the rods 34, 35, 36, 41, 42 and 43 is substantially the same length and slightly less than the distance between end plates 15 and 16. These six rods are also shown centered on the circle essentially half way between inner conductor 13 and outer conductor 14. These specific dimensions and locations are especially convenient from the standpoint of fabrication and design, but may be varied to eifect different electrical properties. The separation between end plates 15 and 16 corresponds substantially to a quarter wavelength for a frequency within the transmission band of the structure. The difference in radi between inner conductor 13 and outer conductor 14 is preferably less than a half wavelength for energy of wavelengths within the transmission band.

The impedances of the rods for a particular bandwidth can be obtained from published formulas. The formulas used for the reduction to practice of the specific embodiment described were obtained from Matthaei, G. L., "Interdigital Bandpass Filters," Transactions, MTT vol. M'IT-lO, November 1962, pp. 479-491. The lengths and end loading capacitance from the ends of the respective rods to the opposite end walls is typically selected so as to produce resonance at the pass band center frequency (approximately 690 mHz. for the filter whose characteristies are set forth in FIG. Actual rod impedances primarly afiect the Q of a filter. The pass band and stop band response are largely a function of 'the ratio of capacitance between the rods and ground plane to the capacitance between the rods. The coupling between the rods is both electrie and magnetic. Because the ods are interdigitalized, the electric and magnetc coupling are additive.

Plate 17 provides both mechanical stiffness and two electrical functions. It shelds input rod 31 from output rod 21. Without this shelding there would be spurious coupling which could distort the pass band rejection. Plate 17 also provides a ground plane for input rod 21 and output rod 31 such that the desired impedance of the input rod can be achieved at a mechanically convenient radial distance from the center line by varying the distance from the rod to the plate 17.

Referring to FIG. 5, there is shown a graphical representation of nsertion loss as a function of frequency for an exemplary embodiment of the invention operative to transmit a wide range of frequencies in the UHF frequency band. The invention is not limited to this frequency range. For low frequency, spacers may be added to prevent the rods from moving or suitably held in place by foamed dielectric material. The physical length of the conducting rods may be reduced by using dielectric loading; that is, filling the annular cavity with low-loss high dielectric material. For narrow band filtering, terminals 11 and 12 may reside in the same end plate and the other end of one or both conducting rods connected 'to an inner terminal grounded to the other end plate. As an example of the wide band performance available at other frequencies, an embodiment of the invention has been constructed in which the nsertion loss is less than a decibel over a full octave transmission range from one gc. to two gc. with the skirts of the transmission band so steep that the attenuation is 35 decibels down 0.4 gc. away from one and two gc., respectively.

There has been described a novel filter having a number of advantages over conventional interdigital filters. Size and form factor is improved. It may be made with low cost Construction using parts that are standard for any frequency percentage bandwidth, requiring only changes in length to accommodate the different frequency ranges. The last advantage makes the invention especially adaptable for use as a tunable filter by sliding the fingers and one end plate relative to the other fingers and end plate.

The number of rods and position of each may be varied. In general an increase in the number of rods is accompanied by a slight increase in the transmission band insertion loss and a much greater increase in the stop band nsertion loss to nhance selectivity,

It is evident that those skilled in the art may now make numerous modifications of and departures from the specific embodiment described herein without departing from the inventive concepts. consequently, the invention is to be construed as limited solely by the spirit and scope of the appended claims.

What is claimed is:

1. Selective transmission apparatus comprising,

conducting means dening an annular cavity bounded by coaxial inner and outer conductors separating first and second end plates,

a conducting plate intercoupling said inner conductor and said outer conductor in a radial plane extending between said first and second end plates to establsh reference potential throughout said radial plane,

first and second terminal pairs for intercoupling said annular cavity with external apparatus,

and means including conducting rods within said annular cavity generally parallel to the axis of said coaxial inner and outer conductors for intercoupling said first terminal pair with second terminal -pail',

alternate ones of said rods being connected to said first end plate and extending almost to said second end plate,

remaining ones of said rods being connected to said second end plate and extending almost to said first end plate,

the frequency response characteristic between said first and second coaxial terminal pairs being related to the position of said conducting rods.

2. Selective transmission apparatus in accordance with claim 1 wherein the separation between said first and sec ond end plates corresponds substantially to a quarter wavelength at a frequency within the band of wavelengths of energy transmitted between said first and sec ond terminal pairs and the separation between said inner and outer conductors is less than that between said end plates.

3. Selective transmission apparatus comprising,

conducting means defining an annular cavity bounded by coaxial inner and outer eonductors separating first and second end plates by a distance less than the separation between said inner and outer conductors,

a conducting plate intercouplng said inner conductor and said outer conductor in a radial plane extending between said first and second end plates to establish reference potential throughout said radial plane,

a first coaxal terminal pair in said first end plate,

a second coaxial terminal pair in said second end plate,

and means including conducting rods within said annular cavity generally parallel to the axs of said coaxal inner and outer conductors for intercoupling said first coaxial terminal pair with said second coaXial terminal pair,

alternate ones of said rods being connected to said first end plate and extendng almost to said second end plate,

remaining ones of said rods being connected to said second end plate and extending almost to said first end plate,

the frequency response characteristic between said first and second coaxial terminal pail's being related to the position of said conducting rods.

4. Selective transmission apparatus in accordance with claim 3 wherein the separation between said first and second end plates corresponds substantially to a quarter wavelength at a frequency within the band of wavelengths of energy transmitted between said first and second coaxial terminal pairs.

5. Selective transmission apparatus in accordance with claim 4 wherein each of said rods extend nearly the length of said cavity,

a first of said rods being connected to the inner terminal of said first coaxial terminal pair,

a second of said rods being connected to the inner terminal of said second coaxial terminal pair.

6. Selective transmission apparatus in accordance With claim 5 wherein said rods are symmetricaily disposed about the aXs of said inner and outer conductors,

half of the remaining ones of said rods being connected to one of said end plates and the other half being connected to the other of said end plates.

7. Selective transmssion apparatus in accordance with claim 6 wheren said first and second rods are immediately adjacent to and separated by said conducting plate.

References Cited UNITED STATES PATENTS HERMAN KARL SAALBACH, Primary Exam'ner.

C. BARAFF, Assistant Exam'ner. 

